The present invention relates to a surface inspection method and a surface inspection system for detecting foreign objects on a surface of a substrate such as a wafer.
In the manufacture of a semiconductor device, fine foreign objects spread on a surface of a substrate such as a wafer exerts extensive influence on the quality of the product. For this reason, surface inspection is performed on the surface of the substrate in the process to manufacture the semiconductor device. The semiconductor devices with increasingly high density are now produced, and the manufacturing process is also becoming more and more complicated. Moreover, various types of films are formed on the surface of the wafer.
A surface inspection system detects foreign objects by projecting an inspection light to the surface of the substrate and by receiving a scattered reflection light from the substrate using a photodetector. In the reflection light from the substrate, reflection characteristics vary according to film quality and film thickness of the film formed on the substrate surface. For this reason, in order to improve an S/N ratio and to increase detection accuracy of the surface inspection, it is necessary to set an inspection condition to correspond to film quality.
The reflection light detected by the photodetector contains not only the scattered reflection light from foreign objects but also a scattered reflection light from the surface of the substrate itself. The scattered reflection light of the substrate surface itself is turned to noise and decreases the detection accuracy in the detection of foreign objects.
JP-A-2001-208697 discloses a surface inspection system, which provides higher detection accuracy by removing the scattered reflection light from the substrate surface itself.
The invention of JP-A-2001-208697 is based on the fact that, when a laser beam is projected to a substrate and it is reflected, condition of polarization is changed due to scattered reflection from the substrate surface. When an S-polarized laser beam is projected to the substrate, an elliptically polarized laser beam is reflected from the substrate surface. A polarizing plate is arranged on a photodetection system, and the scattered reflection light is received through the polarizing plate. By rotating the polarizing plate, a condition is obtained where a photodetection level of the photodetector is at the lowest (i.e. a condition where the reflection light on the substrate surface is at the lowest). The polarizing plate is positioned at a rotating position under this condition, and surface inspection of the substrate is carried out.
The scattered reflection light from the substrate surface is reduced by the polarizing plate, and a ratio of the scattered reflection light from foreign objects to the scattered reflection light from the substrate surface (i.e. S/N ratio) is increased, and this leads to higher detection accuracy.
It is already known that the scattered reflection light on the substrate surface contains a plurality of polarization components depending on the types of the films formed on the substrate surface. In this case, by such method as to eliminate the influence of the reflection light by finding the rotating position of the polarizing plate where the scattered reflection light from the substrate surface is at the lowest as in the case of the conventional type surface inspection system, there are two or more rotating positions of the polarizing plate where the scattered reflection light from the substrate surface is at the minimal value. Further, it is already known that an amount of detected light of the scattered reflection light from foreign objects is also influenced by the rotating position of the polarizing plate. In this respect, sufficient detection accuracy cannot be attained in the surface inspection of foreign objects by simply rotating the polarizing plate and by performing inspection under the condition where the scattered reflection light on the substrate surface is at the lowest.